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Thru-hull plumbing inquiry

Discussion in 'Marine Plumbing and Sanitation' started by lynn silverman, Sep 10, 2017. Add this thread to a FAQ

  1. lynn silverman

    lynn silverman

    Joined Jul 11, 2007
    84 posts, 0 likes
    Hunter 44AC
    US Marina Del Rey, CA Marina Del Rey
    Looking for learned opinion on the practice of screwing seacocks with female NPT fitting to mushroom thru-hull's that have a male NPS thread pattern.

    Every instinct I have tells me that it is wrong and frankly dangerous below the waterline however my 2007 H44AC has that on all of it's thru-hulls and I've noticed it on other boats as well. I can't find online any seacocks that have NPS thread patterns so I am pretty sure my observations are accurate.

    I know you can avoid the issue by using something like a tri-flange seacock but the installation time is probably triple that of the practice I noted above and therefore you get what you pay for from a factory production boat.

  2. lynn silverman

    lynn silverman

    Joined Jul 11, 2007
    84 posts, 0 likes
    Hunter 44AC
    US Marina Del Rey, CA Marina Del Rey
    as a follow up, I found this evidence of it being a non-conforming condition

    • “Threads used in seacock installations shall be compatible (e.g., NPT to NPT, NPS to NPS).”

    This entry, which specifically prohibits the mixing of incompatible thread types, was added to the standard in 2008. While it’s inconceivable to me that any experienced boat builder or mechanic would knowingly attach plumbing components whose threads were incompatible, it’s an all-too-frequent occurrence; thus the specific prohibition.

    Very informative column...

  3. jamhass


    Joined Sep 9, 2017
    9 posts, 0 likes
    Valiant 40
    US Livermore CA Coos Bay -- Currently La Paz
    When you say "Seacocks" with NPT threads, I assume you are really talking about NPT valves -- probably ball valves) screwed directly to the thru-hull, with a backing nut securing the thru-hull to the hull (preferably with a backing plate.

    All the seacocks I have seen or worked with have an NPS thread for the mushroom and an NPT thread on the inboard side.

    Besides the thread incompatibility, there is a serious issue in the fact that you are relying on the very thin wall of the mushroom for strength. This becomes even more serious as the thread incompatibility keeps the valve from seating very far down the mushroom, making for a large moment arm in the event that a sideways force is placed on the valve, thus increasing the probability of a failure.

    It's not that much harder to install a proper seacock, especially if you use a fiberglass mount to hold the mounting screws rather than drilling thru the hull.

  4. njlarry


    Joined Sep 23, 2009
    1,275 posts, 110 likes
    O'Day 34-At Last
    US Rock Hall, Md
    I prefer to use the adapter fittings made by Groco. The nps threads on the mushroom fits the nps threads on the adapter which mounts to the inside hull. The ball valve then fits the npt on the in hull side of the adapter. Also allows for easier replacement of ball valves in the future.
    Nice you tube by Wayne Canning is well worth watching.

  5. Bill Roosa

    Bill Roosa

    Joined Jun 6, 2006
    6,933 posts, 133 likes
    Hunter 40.5
    US Harrington Harbor North, MD
    While a seacock may sound great a mushroom thruhull and ball valve are much easier to replace when the ball valve dies. I do not recall ever hearing of a mushroom thruhull failing. also only use bronze ball valves with bronze thruhulls

  6. twalker H260

    twalker H260

    Joined Dec 2, 2003
    325 posts, 15 likes
    Hunter 260
    CA winnipeg, Manitoba
    If i recall correctly there are some thruhulls that use a combination thread allowing either a NPT or NPS ball valves to be utilized on the mushroom thruhull. As others have mentioned Groco has an adaptor that screws on to the NPS mushroom thruhull and then matches threads with the NPT ball valve. Much better solution than the combination thread thruhull and you can be sure the thruhull won't break its seal with the hull if you have to replace the ball valve.

    Maine Sail has a very useful guide on how to attach the adaptors to the hull and thruhull. - I did mine following his method several years ago - no issues since.

    One thing not frequently mentioned however is that not all thruhulls seem to have accuratly machined threads. I had to replace the mushroom thruhulls in order to get some that would thread on to the Groco adaptors - Test the fit early on!

  7. lynn silverman

    lynn silverman

    Joined Jul 11, 2007
    84 posts, 0 likes
    Hunter 44AC
    US Marina Del Rey, CA Marina Del Rey
    IMG_0148.jpg thanks everyone, good comments, I've come to the conclusion that it would be prudent to upgrade to ABYC standards... shame on Hunter for commissioning boats like this. FYI, this is how Hunter's installation looks after 10 years.. NPS to NPT using an inline ball valve, rigid pipe to an unsupported plastic strainer...

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